Exactly - the chickens of "yester-year" were not nearly what the modern breeds have been morphed into. Chickens in those days, for example, had production rates that were lower than that of today's modified specimens - that extra production requires additional nutritional support that was not needed by/available to the chickens of old, which is reflected in the lower production. Additionally, the birds then that were able to sustain themselves were doing so in a setting that is very different from what is available to most folks these days with regards to appropriate forage in the amounts that would provide sufficient nutrition. The ecological diversity was much greater before we humans came up with all our great ideas as to how we could "improve" things.